1. Technical Field
The present disclosure relates to the field of underwater acoustic (UWA) communications. More particularly, the present disclosure relates to orthogonal frequency division multiplexing (OFDM) systems for UWA communications.
2. Background Art
Underwater acoustic (UWA) communication (the sending and receiving of acoustic signals underwater) is an inherently difficult and complex process. The unique characteristics of water as a propagation medium contribute to the problematic nature of UWA communication. Thus, due to factors like multi-path propagation, time variations of the channel, it is necessary to account for, inter alia, small available bandwidth and strong signal attenuation. Moreover, slow propagation speeds associated with acoustic signals lead to significant Doppler shifts and spreading. Thus, UWA communication systems are often times limited by reverberation and time variability beyond the capability of receiver algorithms.
Unlike the development of wireless networks over radio channels, the development of underwater communication systems has occurred at a much slower pace. See, e.g., M. Stojanovic, “Recent advances in high-speed underwater acoustic communications,” IEEE Journal of Oceanic Engineering, Vol. 121, No. 2, pp. 125-136, April 1996; D. B. Kilfoyle and A. B. Baggeroer, “The state of the art in underwater acoustic telemetry,” IEEE Journal of Oceanic Engineering, Vol. 25, No. 1, pp. 4-27, January 2000. The last two decades have witnessed only two fundamental advances in underwater acoustic communications. One significant advance is the introduction of digital communication techniques, namely, non-coherent frequency shift keying (FSK), in the early 1980's. See, e.g., D. J. Garrood, “Applications of the MFSK acoustical communication system,” in Proc. of OCEANS, Boston, Mass., 1981; and A. Baggeroer, D. E. Koelsch, K. von der Heydt, and J. Catipovic, “DATS—a digital acoustic telemetry system for underwater communications,” in Proc. of OCEANS, Boston, Mass., 1981. The other significant advance is the application of coherent modulation, including phase shift keying (PSK) and quadrature amplitude modulation (QAM) in the early 1990's. See, e.g., M. Stojanovic, J. A. Catipovic, and J. G. Proakis, “Adaptive multichannel combining and equalization for underwater acoustic communications,” Journal of the Acoustical Society of America, Vol. 94, No. 3, pp. 1621-1631, 1993; and “Phase-coherent digital communications for underwater acoustic channels,” THEE Journal of Oceanic Engineering, Vol. 19, No. 1, pp. 100-111, January 1994.
Existing (phase-coherent) UWA communication has mainly relied on serial single-carrier transmission and equalization techniques over the challenging UWA media. See, e.g., D. B. Kilfoyle and A. B. Baggeroer, “The state of the art in underwater acoustic telemetry,” IEEE Journal of Oceanic, Engineering, Vol. 25, No. 1, pp. 4-27, January 2000. However, as data transfer rates increase, symbol durations decrease, causing a greater number of channel taps in the baseband discrete-time model (easily on the order of several hundreds taps). This level of signal degradation poses great challenges for the channel equalizer. Thus, data transfer rates for single-carrier UWA communication techniques are effectively limited by the required receiver complexity.
Multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM) has prevailed in recent broadband wireless radio applications due to the low complexity of receivers required to deal with highly dispersive channels. For example OFDM has been the “workhorse” modulation present in a number of practical broadband wireless systems, notably wireless local area networks (IEEE 802.11a/g/n) (See A. Doufexi, S. Armour, M. Butler, A. Nix, D. Bull, J. McGeehan, and P. Karlsson, “A comparison of the HIPERLAN/2 and IEEE 802.11a wireless LAN standards,” IEEE Communications Magazine, Vol. 40, No. 5, pp. 172-180, May 2002), and wireless metropolitan area networks (IEEE 802.16) (See IEEE Standard 802.16 Working Group, IEEE standard for local and metropolitan area networks part 16; air interface for fixed broadband wireless access systems, 2002). The primary advantages of OFDM over single-carrier schemes is the ability to cope with severe channel conditions, e.g., frequency-selective fading due to multipath propagation without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate intersymbol interference (ISI). Channel equalization using OFDM is further simplified by approximating the effects of frequency-selective channel conditions as a constant for each OFDM sub-channel provided that each sub-channel is sufficiently narrow-band.
These advantages motivate the use of OFDM in underwater environments as well. See, e.g., S. Coatelan and A. Glavieux, “Design and test of a coded OFDM system on the shallow water acoustic channel,” in Proc. of OCEANS, September 1994; B. Kim and I. Lu, “Sea trial results of a robust and spectral-efficient OFDM underwater communication system (Abstract),” The Journal of the Acoustical Society of America, Vol. 109, No. 5, p. 2477, May 1, 2001; and R. Bradbeer, E. Law, and L. F. Yeung, “Using multi-frequency modulation in a modem for the transmission of near-realtime video in an underwater environment,” in Proc. of IEEE International Conference on Consumer Electronics, June 2003. However, as noted above, UWA channels are far more challenging than their radio counterparts. Specifically, with limited bandwidth, UWA channels are wideband in nature due to the small ratio of the carrier frequency to the signal bandwidth. Thus frequency-dependent Doppler drifts are introduced which destroy the orthogonality among OFDM subcarriers.
Table 1 highlights the challenges of multicarrier communication over underwater acoustic channels relative to wireless radio channels, e.g., IEEE802.11a/g, and OFDM based ultra-wideband (UWB) systems:
TABLE ICOMPARISON OF OFDM PARAMETERS IN UNDERWATERACOUSTIC, RADIO, AND UWB CHANNELSExperimentsWirelessOFDMfor this paperLAN [22]UWB [23]Propagation speed c1500m/s3 · 108 m/s3 · 108 m/sBandwidth B12kHz20MHz528MHzCarrier frequency fc27kHz5.2GHz3~10 GHzfrequencyhoppingNarrowband (B/fc < 0.25)widebandnarrowbandwidebandor wideband (B/fc > 0.25)?waveform time compression1.3 · 10−3 for7 · 10−8 for7 · 10−9 foror expansion factor for aυ = 2 m/sυ = 20 m/sυ = 2 m/smoving terminal with speedυ (a = υ/c)Typical multipath spread Td~10ms~500ns~100nsTypical coherence time Tc~1s~5ms~2msOne OFDM symbol duration~85ms4μs0.3μs
The following observations from Table 1 are noted:                1) A common definition of an (ultra) wideband radio is that the system bandwidth exceeds 500 MHz or is greater than 25% of the carrier frequency. Thus, although underwater acoustic channels have limited bandwidth, signaling must be treated as (ultra) wideband.        2) Relative motion between a transmitter and a receiver results in a Doppler-scaled communication signal. The distortion of the signal is proportional to the ratio of the relative speed of the transmitter to the receiver and the propagation speed. Thus, since sound propagates slowly underwater signal compression and/or expansion cannot be ignored for UWA channels.        3) In high-rate wireless radio applications, the symbol block period is small relative to the channel coherence time. Consequently, the channel can be viewed as time-invariant within one block. On the other hand, channel time-variation within one data block is not negligible for underwater applications, and thus it should be explicitly dealt with.        
The existing literature concerning OFDM based UWA communication focuses mostly on conceptual system analysis and simulation based studies. See, e.g., E. Bejjani and J. C. Belfore, “Multicarrier coherent communications for the underwater acoustic channel,” in Proc. of OCEANS, 1996; W. K. Lam and R. F. Ormondroyd, “A coherent COFDM modulation system for a time-varying frequency-selective underwater acoustic channel,” in Proc. of the 7th International Conference on Electronic Engineering in Oceanography, June 1997, pp. 198-203; W. K. Lam, R. F. Ormondroyd, and J. J. Davies, “A frequency domain adaptive coded decision feedback equalizer for a broadband UWA COFDM system,” in Proc. of OCEANS, 1998; and Y. V. Zakharov and V. P. Kodanev, “Multipath-Doppler diversity of OFDM signals in an underwater acoustic channel,” in IEEE International Conference on Acoustics, Speech, and Signal Processing, Vol. 5, June 2000, pp. 2941-2944. Experimental results are far more scarce. See, e.g., S. Coatelan and A. Glavieux, “Design and test of a coded OFDM system on the shallow water acoustic channel,” in Proc. of OCEANS, September 1994; B. Kim and I. Lu, “Sea trial results of a robust and spectral-efficient OFDM underwater communication system (Abstract),” The Journal of the Acoustical Society of America, Vol. 109, No. 5, pp. 2477 et seq., May 1, 2001; and R. Bradbeer, E. Law, and L. F. Yeung, “Using multi-frequency modulation in a modem for the transmission of near-realtime video in an underwater environment,” in Proc. of IEEE International Conference on Consumer Electronics, June 2003.
More recently, several intensive investigations on underwater OFDM communication have been conducted. These investigations include: P. J. Gendron, “Orthogonal frequency division multiplexing with on-off-keying: Noncoherent performance bounds, receiver design and experimental results,” U.S. Navy Journal of Underwater Acoustics, vol. 56, no. 2, pp. 267-300, April 2006; M. Stojanovic, “Low complexity OFDM detector for underwater channels,” in Proc. of MTS/IEEE OCEANS conference, Boston, Mass., Sep. 18-21, 2006; and B. Li, S. Zhou, M. Stojanovic, and L. Freitag, “Pilot-tone based ZP-OFDM demodulation for an underwater acoustic channel,” in Proc. of MTS/IEEE OCEANS conference, Boston, Mass., Sep. 18-21, 2006.
An example of current state-of-the-art technology for underwater communication is the Micro-Modem from Woods Hole Oceanographic Institution (WHOI) that supports a phase-shift-keying (PSK) mode at data rates of 300-5000 bps. The WHOI technology is described in the following publication, the contents of which are incorporated by reference herein in their entirety: L. Freitag, M. Grund, S. Singh, J. Partan, P. Koski, and K. Ball, “The WHOI Micro-Modem: An acoustic communications and navigation system for multiple platforms,” in Proceeding of OCEANS, Washington D.C., 2005. Descriptions of other various existing approaches to UWA communications can be found, for example, at: M. Stojanovic, “Recent advances in high-speed underwater acoustic communications,” IEEE Journal of Oceanic Engineering, Vol. 121, No. 2, pp. 125-136, April 1996; and, D. B. Kilfoyle and A. B. Baggeroer, “The state of the art in underwater acoustic telemetry,” IEEE Journal of Oceanic Engineering, Vol. 25, No. 1, pp. 4-27, January 2000.
Current UWA communication systems, including specifically current OFDM based UWA communication systems, fail to adequately mitigate frequency-dependent Doppler drifts, thereby significantly limiting both the range and application of such systems. These and other disadvantages and/or limitations are addressed and/or overcome by the apparatus, systems and methods of the present disclosure.